Vibratory compactors may include a plate or roller that is oscillated or vibrated to impose compaction forces on a densifiable strata, such as ground soil, roadway base material, or paving material. In some instances, an engine or hydraulic motor controllably rotates at least one eccentric mass to impart vibratory motion at a particular frequency to the surface contacting plate or roller member. The result is an oscillatory force with the frequency of the speed of rotation, and an amplitude dependent on the mass eccentricity and speed of rotation. Variations on this basic system include multiple eccentric weights and/or shafts such that by changing the phasing of the multiple weights and/or shafts, the degree of force created by the eccentric masses can be varied. Other systems may include masses that are linearly oscillated to create linear excitation of the surface contacting plate or roller drum. Linear excitation may also be created along a particular axis by rotating eccentric mass systems with the use of counter-rotating masses to counteract off-axis vibrations.
In roller-type compactors, propulsion may be provided by simply driving the roller drums like wheels. In plate compactors, the angle of the compaction forces relative to the stratum may be changed to propel the compactor itself. That is, the same energy that is used to compact the stratum may be used to propel the compactor. Once the stratum is compacted to a certain degree, the surface of the stratum will no longer move significantly downward under the force of the vibrations (i.e., the stratum will not significantly compact). Instead, the compaction forces will have the effect of lifting the compactor by pushing off the now-firm stratum. By angling the vibratory energy at a non-perpendicular angle relative to the surface of the stratum, the compaction forces may have the effect of propelling the compactor along the stratum.
While roller-type compactors may utilize less sophisticated forms of vibratory excitation (because the vibratory energy need not be used for propulsion), the rollers may, in the process of compaction, manipulate the densifiable strata to the detriment of the finished stratum. For example, rolling a drum over an uncompacted stratum can create a “bow-wave.” A bow-wave is an upward bulging of the material in front of the roller drum. Bow-wave is the result of the radius of curvature (i.e., diameter) of the roller drum, which may create a tendency for the drum to plow the uncompacted material. The Nijboer quotient describes the tendency of a roller to push or plow material in front of it. The Nijboer quotient may be calculated with the following formula.
  N  =            Axle      ⁢                          ⁢      Load                      (                  Drum          ⁢                                          ⁢          width                )            *              (                  Drum          ⁢                                          ⁢          Diameter                )            
With paving material, a bow-wave can create cracking in the paving material. This cracking may render the finished pavement with a reduced structural integrity and/or decreased durability.
Although certain conditions may prevent and/or repair the cracking (e.g., sufficient heat in the strata), adequate control of these conditions may be difficult. In view this limitation of roller-type compactors, a plate compactor may provide superior compaction with little or no risk of bow-wave because it has an infinite radius of curvature (see Nijboer quotient formula above, which depends on drum diameter). A plate compactor simply compresses the stratum downward with little or no tendency to plow the material in front of the plate.
An additional advantage of plate compactors over roller-type compactors is that, when stationary, plate compactors are less likely to sink into a stratum. A plate compactor spreads its weight more over a stratum so it will not be as likely to sink (e.g., into freshly-laid asphalt, which may still be warm and, therefore, soft) as would the rollers of a roller-type compactor. Rollers may have contact patches that are not as large relative to the size and weight of the machine as those of a plate compactor.
Historically, large scale compactors (e.g., for highway construction) have been limited to roller-type compactors due to the logistics of moving a plate compactor of that size and the lack of maneuverability of available plate compactors. The benefits of a plate compactor (e.g., no bow-wave) may not have outweighed the obstacle of maneuverability.
Some plate compactors have made use of linear excitation for various benefits (e.g., compaction controllability, energy efficiency, etc.), but have not employed the linear excitation for purposes of propulsion. For example, U.S. Pat. No. 6,293,729, issued to Greppmair on Sep. 25, 2001 (the '729 patent), discloses a plate compactor that uses linear excitation. The '729 patent, however, does not disclose that the excitation is used to propel the compactor itself. Rather, the device of the '729 patent includes one or more sets of wheels, which may be driven to provide propulsion for the compactor.
The present disclosure is directed toward solving one or more problems set forth above.